Absorbing agents for water and aqueous liquids and process for their production and use

ABSTRACT

The present invention relates to a powdery, cross-linked polymer absorbing water and aqueous liquids formed of polymerized, unsaturated, acid groups-containing monomers that are present as salts neutralized to the extent of at least 50 mol-% and which optionally comprise further monomers which are copolymerizable with the acid groups-containing monomers as well as water-soluble polymers, the polymer being formed of acidic, polymerized monomers partially neutralized to the extent of 5-30 mol-%, relative to the acid groups-containing monomer portion. The polymer has a high retention, high liquid absorption under pressure, high swelling pressure, and a low content of soluble components. The present invention further relates to a process for the production of said polymer and to its use as a component in sanitary articles which absorb body fluids and in wound dressings, in current-conducting and light-transmitting cables, as soil conditioners, as a component in packaging materials and in depot materials for the controlled release of active substances.

The present invention relates to water-insoluble, water-swellablepolymers having a high absorption capacity for water and aqueousliquids, which are based on cross-linked, hydrophilic homopolymers andcopolymers, as well as on graft polymers of ethylenically unsaturated,acid groups-containing, polymerizable monomers. The present inventionfurther relates to the production method and the use of these polymers.

Polymers absorbing large amounts of aqueous liquids, in particular bodyfluids such as urine, are known as superabsorbent polymers.

The absorbents are produced by radical polymerization under preferreduse of monoethylenically unsaturated carboxylic acids, such as acrylicacid and its alkali salts, in aqueous solution, or according to themethods of inverse suspension or emulsion polymerization which aredescribed in U.S. Pat. No. 4,286,082, DE 27 06 135, U.S. Pat. No.4,340,706, DE 37 13 601, and DE 28 40 010.

Polymers having different absorption properties can produced byselection of the monomer composition, cross-linkers, and polymerizationconditions and processing conditions for the polymer gel. Additionalpossibilities are offered by the production of graft polymers, forexample, by using chemically modified starch, cellulose, and polyvinylalcohol according to DE 26 12 846, and by aftertreating the polymer gelsor powdery resins by secondary surface cross-linkage of the polymerparticles, for example, according to DE 40 20 780 C1.

For application of the polymers in hygienics and the sanitary field,polymers are produced whose neutralization degree ranges between 50 and80 mol-%, relative to the polymerized, acid groups-containing monomerunits, so that hydrogels are formed which have a skin neutral effectwhen used.

The degree of neutralization may be adjusted in different manners,neutralization or partial neutralization of the acidic monomersfrequently being carried out prior to polymerization. However,neutralization or partial neutralization of a hydrogel comprisingacidic, polymerized monomer units is also known.

According to EP 205 674 A1 advantageously fully acidic polymers areproduced at temperatures ranging from 0 to 100° C., preferably 5 to 40°C., which are adjusted by subsequent partial neutralization of thehydrogel. The polymers stand out for an improved gel strength andabsorption capacity, as well as for a small portion of water-solublepolymers.

According to U.S. Pat. No. 5,145,906 and EP 530 438 B1 polymer gels ofacrylic acid together with water-soluble, hydroxyl groups-containingpolymers are produced at temperatures ranging from 5 to 20° C. underadiabatic conditions without neutralization of the monomers;subsequently the polymer gels are reduced in size and partially orcompletely neutralized by aqueous bases, comminuted again in thepresence of a secondary cross-linking agent, and subjected to a heattreatment.

The mentioned methods are disadvantageous;, because polymerization ofthe monomer solution, as is demonstrated in EP 467 073 A1, takes placevery slowly so that it can only be carried out discontinuously in abatch method. Increasing the amount of initiator or elevating thereaction temperature affects the desired polymer properties.Furthermore, it is difficult to comminute the entirely acidic polymergel, and neutralization takes place slowly because of the procedurewhich is governed by diffusion. In this connection, excess base remainsin the surface region of the polymer particles, while still reactive,polymerized, acid groups-containing monomer units are present in the gelinterior where they are not available for subsequent cross-linkage inthe surface region.

EP 467 073 A1 describes polymers which are obtained at a controlledpolymerization temperature in the range of 20 to 70° C., in particularat 55 to 60° C., within 5 hours. According to the described method thepolyacrylic acid is then neutralized to a degree of 72%. The dry andground polymer is then subjected to the subsequent cross-linkingprocess. However, if liquids are absorbed under pressure, the polymersobtained according to this process have an only limited liquidabsorption capacity. Additionally, after liquid absorption, the knownpolymers have a substantial rewet behavior which is disadvantageous, inparticular when they are used in hygienic articles.

Accordingly, there is the object to provide polymers with improvedabsorption properties, in particular with improved liquid absorptionunder pressure load and improved rewet behavior, and a process for theirproduction under reduction of the polymerization period and improvementof the subsequent cross-linkage.

This object is achieved by a powdery, cross-linked polymer absorbingwater and aqueous liquids which is formed of a) 55-99.9%-wt. of at leastone polymerized, unsaturated, polymerizable, acid groups-containingmonomer which is present as a salt neutralized to the extent of at least50 mol-%, b) 0-40%-wt. of a polymerized, unsaturated monomer which iscopolymerizable with the monomer according to a), c) 0.01-5.0%-wt. of atleast one cross-linking agent, and d) 0-30%-wt. of a water-solublepolymer, with the sum of components a)-d) amounting to 100%-wt., whichpolymer is characterized by the fact that it is formed of acidic polymergels partially neutralized to the extent of 5-30 mol-%, relative to theacid groups-containing monomer portion, and that it has

    ______________________________________                                        a retention for a 0.9% aqueous NaCl-solution of at least 25                     g/g polymer,                                                                  a liquid absorption under a pressure of 50 g/cm.sup.2  of at least           25 g/g polymer,                                                               a swelling pressure after 20 minutes of at least 700 g, rela-                 tive to 1 g polymer,                                                          a maximum soluble content after 16 h of 10%-wt., prefera-                     bly a maximum of 3%-wt., and                                                  a maximum rewet of 2.0 g.                                                    ______________________________________                                    

The object is further achieved by a process for the production of thispolymer, which is characterized by the fact that an aqueous solutionhaving a maximum monomer content of 30% by weight, relative to the totalsolution, is produced; the unsaturated, polymerizable, acidgroups-containing monomers; are neutralized with bases to the extent of5.0-30 mol-%, preferably 10-20 mol-%, prior to polymerization; thepolymerization under radical conditions at a temperature ranging from5-30° C., preferably 8-15° C., is initiated by chemical initiatorsand/or UV-irradiation and continued adiabatically; the polymer gel issubjected to a comminution after coarse grinding, and is neutralizedwith bases such that the polymer comprises a content of polymerized,neutralized, acid groups-containing monomer units of at least 50 mol-%;the polymer gel is dried to a water content of ≦10%-wt.; and that thedry and ground polymer is reacted at a temperature in the range of140-240° C. with at least one bi- or multifunctional cross-linkerreactive with acid groups.

Most surprisingly, we have found that polymers having a high retentionvalue and high absorption capacity under a constant or increasingpressure, and low soluble polymer components are obtained, if thepolymerization is carried out with a solution wherein a small amount ofthe acid groups-containing monomers have been neutralized by theaddition of bases, instead of using a completely acid polymer solution.The range of partial neutralization amounts to about 5-30 mol-%,preferably 5-20 mol-%, and most preferably 5-10 mol-%, relative to acidgroups-containing monomers.

According to the present invention water-soluble, monoethylenicallyunsaturated mono- and dicarboxylic acids, such as acrylic acid,methacrylic acid, ethacrylic acid, crotonic acid, sorbic acid, maleicacid, fumaric acid, itaconic acid, as well as vinylsulfonic acid;acrylamido- and/or methacrylamidoalkyl sulfonic acids, such as2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyl oxyethanesulfonic acid, 4-vinylbenzene sulfonic acid, allyl sulfonic acid,vinyltoluene sulfonic acid, vinylphosphoric acid, and vinylbenzenephosphonic acid, are used as polymerizable, unsaturated, acidgroups-containing monomers a).

Acrylic acid is the preferred water-soluble, unsaturated carboxylicacid. The proportion of other unsaturated carboxylic acids in thepolymer, in addition to acrylic acid, may amount to up to 50%-wt.

Water-soluble, monoethylenically unsaturated monomers, such asacrylamide, methacrylamide, N-alkylated (meth)acrylamides, N-methylol(meth)acrylamide, N-vinyl amides, N-vinyl formamide, N-vinyl acetamide,and N-vinyl-N-methylacetamide, N-vinyl-N-methyl-formamide,vinylpyrrolidone, as well as hydroxyalkyl (meth)-acrylates, such ashydroxyethyl acrylate and (meth)acrylic acid esters of polyethyleneglycol monoallyl ether, and allyl ethers of polyethylene glycols areused as monomers b).

Acrylic acid and methacrylic acid esters, such as ethyl acrylate andmethyl acrylate, vinyl acetate and styrene are also used in limitedamounts as monomers b) with a low solubility in water. The maximumportion of these poorly or sparingly water-soluble monomers amounts to10%-wt., relative to the sum of all monomers.

The mentioned monomers are used to produced homopolymers or copolymerswith at least two monomers in any desired combination. The monomermixture may additionally comprise a content of 0-30%-wt. ofwater-soluble polymers d), relative to the components of the monomersolution. A synthetic polymer or copolymer and/or a natural polymerand/or a derivative of a natural polymer may be used as water-solublepolymer. Examples thereof include water-soluble homo- and copolymers ofthe above-mentioned monomers, such as polyacrylic acid, partiallysaponified polyvinyl acetate, polyvinyl alcohol, polyalklyene glycol,starch, starch derivatives, graftpolymerized starch, cellulose andcellulose derivatives, such as carboxymethylcellulose,hydroxymethylcellulose, as well as galactomannan and its oxalkylatedderivatives.

The aqueous monomer solution comprises at least one cross-linking agentc) in an amount of 0.01-5.0%-wt., preferably 0.1-2.0%-wt., relative toall portions of components a), b), and c) of the monomer solution. Anycompound may be used as cross-linker which comprises at least twoethylenically unsaturated double-bonds, or one ethylenically unsaturateddouble-bond and one functional group reactive towards acid groups, orseveral functional groups reactive towards acid groups. Examples thereofinclude: methylenebisacrylamide, acrylates and methacrylates of polyols,such as butanediol diacrylate, hexanediol dimethacrylate, polyethyleneglycol diacrylate, and trimethylolpropane triacrylate, and/or theacrylates and methacrylates of the oxalkylated mentioned polyols, suchas of oxalkylated trimethylolpropane and oxalkylated pentaerythritol.Cross-linking agents of this type are known under the trade namesSartomer and Craynor (Crayvalley Kunstharze GmbH, 47918 Tonisvorst,Germany), of which Sartomer 415, Sartomer 454, Sartomer 494, Sartomer610, and Craynor 435 are particularly usable; moreover, diesters andpolyesters of polyols and oxethylated polyols with unsaturatedmonocarboxylic acids and/or polycarboxylic acids, such as (meth)acrylicacid esters of 1,2-propylene glycol, pentaerythritol, glycerol andpolyglycerol, as well as monoesters of unsaturated alcohols andethoxylated, unsaturated alcohols with unsaturated monocarboxylic acidsand/or monocarboxylic acids, such as allyl acrylate and methacrylate,monoallyl maleate, allyl polyethylene glycol ether acrylate andmethacrylate, allyl itaconate, allyl polyethylene glycol etheritaconate, and monoallyl polyethylene glycol ether maleate; additionallydiallyl acrylamide, diallyl phthalate, diallyl adipate, triallylcitrate, and trimonoallyl polyethylene glycol ether citrate; moreover,allyl ethers of diols and polyols and their oxethylates, such as diallylethers of ethylene glycol, diethylene glycol, polyethylene glycol,triallyl ethers of glycerol, oxethylated glycerol, trimethylolpropaneand oxethylated trimethylolpropane, tetraallylethers of pentaerythritoland oxethylated pentaerythritol, as well as tetraallyloxyethane, andpolyglycidyl ether, for example, ethylene glycol diglycide ether andglycerol glycidyl ether. Additionally, amines and/or their salts andamides with at least two ethylenically unsaturated alkyl groups, such asdi and triallylamine and tetraallylammonium chloride.

The usual initiators are used to initiate the radical polymerization,for example, peroxo and azo compounds, preferably water-soluble and/ordissociating peroxo and azo compounds, such as tert.-butyl hydroperoxideand 2,2'-azobis(2-methylpropionamidine) dihydrochloride; as well asredox systems formed of sodium and potassium peroxomonosulfate, sodiumand potassium peroxodisulfate, and hydrogen peroxide with sodium andpotassium sulfite, sodium and potassium formamidine sulfinate, andascorbic acid.

When the redox systems are used, the oxidant is preferably preparedfirst and the reducing agent is added afterwards. Particularly in caseof continuous polymerization, initiation is effected throughphotocatalysis with ultraviolet light and the known sensitizers.

The acid groups-containing monomers are preferably neutralized prior tothe addition of the other components of the monomer solution, andpreferably by preparing the base first. Suitable bases include alkalihydroxides, ammonia, and aliphatic, primary and secondary amines, aswell as alkali carbonates and alkali hydrogen carbonates. The alkalihydroxides, sodium hydroxide and potassium hydroxide, as well as ammoniaand soda are preferred.

Prior to polymerization, the monomer solution is cooled to a temperatureranging from 5-30° C., preferably 8-20° C. After initiation,polymerization takes place without inhibitory effects, i.e., withoutdelay both with respect to the starting phase and in the further courseof polymerization; this is a surprising fact and differs from the knownmethods. Polymerization is carried out in the discontinuous batch methodor, advantageously, in continuous manner, for example, on the beltreactor.

The formed polymer gel is then subjected to coarse grinding andcomminution by means of conventional tearing and/or cutting tools.Comminution is preferably carried out by means of a cutting extruder viaterminal breaker plates whose openings have a diameter ranging from 2-20mm, preferably 5-15 mm, and most preferably 8-13 mm.

Subsequent neutralization of the comminuted polymer gel is carried outwith the bases mentioned for partial neutralization, and again sodiumhydroxide solution, potassium hydroxide solution and/or ammonia or soda,e.g., as Na₂ CO₃.10H₂ O or as aqueous solution, are used. Neutralizationis carried out in simple mixing units, for example, in a rotary drum orin a Drais-mixer, and the aqueous solution of the bases is introduced,for example, by means of nozzles or spray injectors. Neutralization iscarried out until at least 50 mol-%, preferably 60-80 mol-%, of the acidgroups-containing, polymerized monomer units are present as salts. Thesubsequent neutralization may also be effected during comminution of thegel, for example, during size reduction in the extruder.

Owing to the slight partial neutralization of the monomer lye, thepolymer gel's affinity to the base, preferably alkali lye, is improvedto such an extent that a simple mixing unit is generally sufficient tocarry out the further neutralization within a short period of time; thusmechanical damage of the polymer gel is prevented to a large extent.

The polymer get is dried to a water content in the range of 5-20%-wt.,preferably of ≦10%-wt., at temperatures in the range of 100-190° C.Subsequently, the dry product is ground into a polymer powder of aparticle size in the range of 20-3,000 μm, preferably 150-850 μm.Subsequent cross-linkage of the polymer takes place on the surface ofthe dry polymer particles using at least one bi- or multifunctionalcross-linker reacting with acid groups, preferably carboxyl groups,which is preferably applied in the form of a hydrous solution. Suitablesecondary cross-linking agents include polyols, such as ethylene glycol,1,2-propylene glycol, 1,4-butanediol, glycerol, di- and polyglycerol,pentaerythritol, the oxethylates of these polyols, as well as theiresters with carboxylic acid or carbonic acid. Advantageously, anesterification catalyst, e.g., p-toluenesulfonic acid or phosphoric acidis added. Further suitable cross-linkers include di- and polyglycidylethers of polyols and polyethylene glycols. Such compounds arecommercially available under the trade name Denacol (Nagase (Europe)GmbH, Dusseldorf).

Subsequent cross-linkage is carried out at temperatures ranging from150-250° C., preferably 150-200° C., in a mixing unit, for example, aNara-mixer.

The production method according to the present invention providespolymers standing out for considerably improved properties as comparedto the conventional products. They have a retention in 0.9% aqueousNaCl-solution of at least 25 g/g polymer, preferably of 30 g/g, mostpreferably at least 33 g/g polymer. Under a pressure load of 50 g/cm²,they achieve a liquid absorption of at least 25 g/g polymer, preferablyat least 27 g/g polymer. The polymers according to the presentinvention, which are swollen by the absorption of water or aqueousliquids, have a high permeability for these liquids in swollencondition, even under pressure. Therefore, good distribution of theliquids is achieved, making full use of the absorption capacity of thepolymers, when liquids turn up successively and repeatedly.Additionally, only very small amounts of the absorbed aqueous liquid arereleased again so that the rewet value amounts to a maximum of 2.0 g,preferably a maximum of 1.0 g. Their capability of absorbing liquids isfurther characterized by a very high swelling pressure of at least 700g, preferably at least 850 g, more preferably of 900 g, and mostpreferably at least 1,000 g, according to the method of Stevens LFRATexture Analyser. The polymers according to the present invention have acontent of soluble polymer portions of below 10%-wt., preferably below5%-wt., and most preferably below 3.5%-wt.

The absorbents obtained according to the process according to thepresent invention may be mixed with further substances. These substancesinclude, for example, fillers, such as chalk, bentonite, or diatomaceousearth, as well as fibrous materials, such as hemp, viscose, or cellulosefibers. Color pigments, ultraviolet absorbents, antioxidants, andagricultural chemicals, such as fertilizers, herbicides, fungicides, andinsecticides may also be used as components. The polymers according tothe present invention may be used in many fields, preferably in hygieneproducts, such as diapers, sanitary napkins, wound patches, andincontinence bags, which serve the rapid and complete absorption ofurine, menstrual blood, and exudation of wounds. In diapers the polymersare present at a weight percentage, relative to the weight amount ofpolymer and fluff, of 15-100%-wt., preferably 35-100%-wt, and mostpreferably of 30-70%-wt. In addition they may be used as component inpackaging inserts to absorb liquids released from food, in cable fillermaterials to prevent ingress of water into seawater cables, as acomponent in current-conducting and light-transmitting cables, and inhorticultural and agricultural substrates which are used as soilconditioners, or as depot materials for the controlled release of activesubstances, or as a component in artificial soils for plant cultivation.The present invention will be demonstrated in the examples that followwherein the following test methods are used:

Retention (TB):

200 mg superabsorber is weighed into a tea bag and immersed intophysiological saline (0.9%-wt. NaCl) for 30 min. After that, the tea bagis hung up at its corners for 10 min. to remove the free water and thencentrifuged in a centrifuge at about 1,400 rpm. The blank reading andthe initial weight are deducted from the resulting weight, and theresult is converted to 1 g (TB g/g).

Absorption under pressure (AAP):

0.9 g superabsorber is evenly distributed in a cylinder having an innerdiameter of 6 cm, its bottom consists of a sieve fabric having a meshsize of 35 μm. A weight of 20 g/cm³ (AAP₀.3 psi) or 50 g/cm³ (AAP₀.7psi) presses on the superabsorber. The cylinder unit is weighed and thenplaced on a ceramic filter plate which immerses to the same level intophysiological saline. After 1 hour, the cylinder unit is reweighed. Thequotient from the weight difference of the cylinder units and theinitial Freight of the superabsorber is taken as absorption value[(W_(after) -W_(before))/initial weight].

Rewet:

The rewet test is carried out according to the instructions in EP 0 631768 A1, p. 19, l. 39 to p. 20, l. 4; however, a 0.9% aqueousNaCl-solution is used instead of the synthetic urine.

Swelling pressure (SP):

Determination of the swelling pressure SP is carried out by means of theStevens L.F.R.A. Texture Analyser, C. Stevens & Son Ltd., LaboratoryDivision, St. Albans AL1 1 Ex Herifordshire, England. 0.500 gsuperabsorber of size fraction 20-50 mesh is weighed into the measuringcylinder having a diameter of 2.7 cm, and 10 ml of 0.9% NaCl-solution isadded. Then, the measuring cylinder is brought up by means of a liftingstage until the distance between the lower edge of the cylindricalmeasuring instrument and the surface of the sample in the measuringcylinder amounts to 12 mm. Through expansion of the gel the measuringcylinder is pressed upwards against a two-way load-sensing cell, and theload is indicated at the device in grams.

Residual monomer (RM):

Determination of the residual amounts of acrylic acid in thesuperabsorber is carried out by gas chromatography, after extractionwith physiological saline up to equilibrium state.

Soluble Contents (SC):

The polymer portion in a superabsorber that can be extracted within 16hours and is related to carboxylic acid is determined according a methoddescribed in EP pat. application 0 205 674, pp. 34-37; however,physiological saline solution is used instead of the synthetic urine.The quantities are given in % by weight, relative to the dry, groundpolymer.

The present invention will be illustrated in greater detail by means ofthe following examples and comparative data:

EXAMPLE 1

80 g acrylic acid and 0.24 g of a triacrylate of an oxethylated (15moles of ethylene oxide) trimethylolpropane are dissolved in 244 g waterand preneutralized with 4.4 g 50% sodium hydroxide solution. After icecooling to 6 to 8° C. and purging with nitrogen to a maximum residualoxygen content of 0.3 ppm, 0.033 g 2,2'-azo-bis(2-methylpropionamidine)dihydrochloride, 0.12 g sodium persulfate, and 0.023 g hydrogen peroxide(35%) are introduced. Then, polymerization is started with 0.003ascorbic acid. After an initiation phase of about 5 min., thetemperature rises constantly and reaches the maximum temperature of 72°C. after about 20 min. Another 15 min. are maintained for secondaryreaction, and then the resultant polymer gel is passed through a meatgrinder. The comminuted mass is then mixed with 57.8 g 50% sodiumhydroxide solution (increase of the neutralization degree to 70%) in adrum having a horizontal agitator shaft, dried to a water content of<10% at 140 to 160° C., and ground to a particle size of >180 to <850μm. The obtained granulate is superficially re-cross-linked at 190° C.using ethylene carbonate (0.5% abs. dry, dissolved in water/acetone).

EXAMPLE 2

80 g acrylic acid and 0.2 g of a triacrylate of an oxethylated (3 molesof ethylene oxide) trimethylolpropane are dissolved in 244 g water andpreneutralized with 8.9 g 50% sodium hydroxide solution. After icecooling to 6 to 8° C. and purging with nitrogen to a maximum residualoxygen content of 0.3 ppm, 0.033 g 2,2'-azo-bis(2-methylpropionamidine)dihydrochloride, 0.12 g sodium persulfate, and 0.023 g hydrogen peroxide(35%) are introduced. Polymerization is started with 0.003 ascorbicacid. After about 20 min., the maximum temperature of 74° C. isachieved. For secondary reaction, another 15 min. are maintained, andthen the resultant polymer gel is passed through a meat grinder. Thecomminuted mass is then mixed with 53.3 g 50% sodium hydroxide solution(increase of the neutralization degree to 70%) in a drum having ahorizontal agitator shaft, dried to a water content of <10% at 140 to160° C., and ground to a particle size of >180 to <850 μm. The obtainedgranulate is superficially re-cross-linked at 190° C. using ethylenecarbonate (0.5% abs. dry, dissolved in water/acetone).

EXAMPLE 3

80 g acrylic acid and 0.16 g trimethylolpropane are dissolved in 244 gwater and preneutralized with 8.9 g 50% sodium hydroxide solution. Afterice cooling to 6 to 8° C. and purging with nitrogen to a maximumresidual oxygen content of 0.3 ppm, 0.033 g2,2'-azobis(2-methylpropionamidine) dihydrochloride, 0.12 g sodiumpersulfate, and 0.023 g hydrogen peroxide (35%) are introduced.Polymerization is then started with 0.003 ascorbic acid. After about 20min., the maximum temperature of 74° C. is achieved. Another 15 min. aremaintained for secondary reaction, and then the resultant polymer gel ispassed through a meat grinder. The comminuted mass is then mixed with53.3 g 50% sodium hydroxide solution (increase of the neutralizationdegree to 70%) in a drum having a horizontal agitator shaft, dried to awater content of <10% at 140 to 160° C., and ground to a particle sizeof >180 to <850 μm. The obtained granulate is superficiallyre-cross-linked at 190° C. using ethylene carbonate (0.5% abs. dry,dissolved in water/acetone).

EXAMPLE 4

80 g acrylic acid and 0.24 g of a triacrylate of an oxethylated (15moles of ethylene oxide) trimethylolpropane are dissolved in 244 g waterand preneutralized with 8.9 g 50% sodium hydroxide solution. After icecooling to 6 to 8° C. and purging with nitrogen to a maximum residualoxygen content of 0.3 ppm, 0.033 g 2,2'-azo-bis(2-methylpropionamidine)dihydrochloride 0.12 g sodium persulfate, and 0.023 g hydrogen peroxide(35%) are introduced. Polymerization is started with 0.003 ascorbicacid. After about 20 min., the maximum temperature of 74° C. isachieved. Another 15 min. are maintained for secondary reaction, andthen the resultant polymer gel is passed through a meat grinder. Thecomminuted mass is then mixed with 53.3 g 50% sodium hydroxide solution(increase of the neutralization degree to 70%) in a drum having ahorizontal agitator shaft, dried to a water content of <10% at 140 to160° C., and ground to a particle size of >180 to <850 μm. The obtainedgranulate is superficially re-cross-linked at 190° C. using ethylenecarbonate (0.5% abs. dry, dissolved in water/acetone).

EXAMPLE 5

80 g acrylic acid and 1 g polyethylene(600)-glycol diacrylate aredissolved in 240 g water together with 1 g Mowiol 5/88 (Hoechst) andpreneutralized with 8.9 g 50% sodium hydroxide solution. After icecooling to 6 to 8° C. and purging with nitrogen to a maximum residualoxygen content of 0.3 ppm, 0.23 g 2,2'-azobis(2-methylpropionamidine)dihydrochloride and 0.24 g hydrogen peroxide (35%) are introduced. Then,polymerization is started with 0.02 g ascorbic acid.

After about 1 5 min., the maximum temperature of 75° C. is achieved.After a 15 min.-downtime for secondary reaction, the resultant polymergel is passed through a meat grinder. The comminuted mass is then mixedwith 190.7 g uncalcined, solid soda (increase of the neutralizationdegree to 70%) in a drum having a horizontal agitator shaft, dried to awater content of <10% at 140 to 160° C., and ground to a particle sizeof >180 to <850 μm. The obtained granulate is superficiallyre-cross-linked at 190° C. using ethylene carbonate (0.5% abs. dry,dissolved in water/acetone).

EXAMPLE 6

80 g acrylic acid and 1 g polyethylene(400)-g glycol diacrylate aredissolved in 240 g water together with 1 g Mowiol 5/88 (Hoechst) andpreneutralized with 26.7 g 50% sodium hydroxide solution. After icecooling to 6 to 8° C. and purging with nitrogen to a maximum residualoxygen content of 0.3 ppm, 0.23 g 2,2'-azobis(2-methylpropionamidine)dihydrochloride and 0.24 g hydrogen peroxide (35%) are introduced. Then,polymerization is started with 0.02 g ascorbic acid. After about 15min., the maximum temperature of 75° C. is achieved. After a 20min.-downtime for secondary reaction, further processing was carried outas described in Example 1; for subsequent neutralization, 35.5 g 50%sodium hydroxide solution was used.

EXAMPLE 7

80 g acrylic acid and 1 g polyethylene(400)-glycol diacrylate aredissolved in 240 g water together with 1 g Mowiol 5/88 (Hoechst) andpreneutralized with 4.5 g 50% sodium hydroxide solution. After icecooling to 6 to 8° C. and purging with nitrogen to a maximum residualoxygen content of 0.3 ppm, 0.23 g 2,2'-azobis(2-methylpropionamidine)dihydrochloride and 0.24 g hydrogen peroxide (35%) are introduced. Then,polymerization is started with 0.02 g ascorbic acid. After about 25min., the maximum temperature of 72° C. is achieved. After a 20min.-downtime for secondary reaction, further processing was carried outas described in Example 1; for subsequent neutralization, 39.9 g 30%sodium hydroxide solution was used.

Comparative Example 1

Polymerization trial without preneutralization degree:

80 g acrylic acid and 0.24 g of a triacrylate of an oxethylated (3 molesof ethylene oxide) trimethylolpropane were dissolved in 244 g water.After ice cooling to 6 to 8° C. and purging with nitrogen to a maximumresidual oxygen content of 0 3 ppm, 0.033 g2,2'-azobis(2-methylpropionamidine) dihydrochloride, 0.12 g sodiumpersulfate, and 0.023 g hydrogen peroxide (35%) were introduced. Then,it was attempted to start the polymerization (as in Example 1) with0.003 g ascorbic acid. This failed, and 0.02 g ascorbic acid wereadditionally added. After a downtime of about 5 min., polymerizationstarted and the maximum temperature of 64° C. was achieved after about30 min. For secondary reaction, another 15 min. were maintained, andthen the resultant polymer gel was passed through a meat grinder. The(comminuted mass was then mixed with 62.2 g sodium hydroxide solution(increase of the neutralization degree to 70%) in a drum having ahorizontal agitator shaft, dried to a water content of <10% at 140 to160° C., and ground to a particle size of >180 to <850 μm. The obtainedgranulate was superficially re-cross-linked at 190° C. using ethylenecarbonate (0.5% abs. dry, dissolved in water/acetone).

                  TABLE 1                                                         ______________________________________                                        Properties of the polymers                                                                                  SP 20/                                                                                      TB AAP                                                                       .sub.0.3 AAP.sub.0.7 120 min.                                                 SC Rewet                             (g/g) (g/g) (g/g) (g) %-wt. (g)                                             ______________________________________                                        Example 1                                                                             34      36      28    850/900                                                                              4.5   0.5                                  Example 2 35 36 29 880/920 5 1.0                                              Example 3 35 36 28 850/890 5 1.0                                              Example 4 35 36 29 860/920 5 1.0                                              Example 5 34 37 30 1040/940  4 0.2                                            Example 6 34 37 30 1120/925  5 1.0                                            Example 7 35 36 29 910/920 4 2.0                                              Comp. Ex. 1 32 34 25 820/850 4 2.5                                            Sanwet IM 31 29 19 577/496 5.2 2.5                                            4000*                                                                         Sanwet IM 35 29.5 16 609/522 5.4 2.9                                          7000*                                                                         Sanwet IM 33 34 20 529/567 2.8 8.5                                            7000*                                                                       ______________________________________                                         *by Hoechst AG                                                           

Table 2

Retentions of acidic polymer gels (about 24% active substance, relatedto acrylic acid, according to Example 2 or Comparative Example 1) withdiffering neutralization degree:

    ______________________________________                                        Example     Neutralization degree (%)                                                                       TB (g/g)                                        ______________________________________                                        Comp. Ex. 1  0                2.04                                              2  5 2.25                                                                     2 10 2.80                                                                     2 20 4.96                                                                     2 30 8.90                                                                   ______________________________________                                    

The data in Table 2 show the extreme increased in the absorptioncapacity of acrylate gels for aqueous liquids in the neutralizationrange of 0 to 30% of the acrylic-acid-polymer gels, on the basis of theretention values of physiological saline.

What is claimed is:
 1. A powdery, cross-linked polymer, comprising:a)55-99.9%-wt. of at least one unsaturated, polymerizable, acidgroup-containing monomer which is present as a salt neutralized to theextent of 5-30 mol-%, b) 0-40%-wt. of at least one unsaturated monomerwhich is copolymerizable with the monomer according to a), c)0.01-5.0%-wt. of at least one cross-linking agent, and d) 0-30%-wt. ofat least one water-soluble polymer,with the sum of components a)-d)amounting to 100%-wt., and the portion of the polymerized acidgroup-containing monomer a) in the polymer is neutralized to the extentof at least 50 mol-%, and the polymer is additionally cross-linked onthe surface and it hasa retention for a 0.9% aqueous NaCl-solution of atleast 25 g/g polymer, a liquid absorption under a pressure of 50 g/cm²of at least 25 g/g polymer, a swelling pressure after 20 minutes of atleast 700 g, relative to 1 g of polymer, a maximum soluble content of3.5-10%-wt after 16 hours, and a maximum rewet of 2.0 g.
 2. The polymeraccording to claim 1, wherein said polymer has a retention for a 0.9%aqueous NaCl-solution of at least 30 g/g polymer, a liquid absorptionunder a pressure of 50 g/cm² of at least 27 g/g polymer, and a swellingpressure after 20 minutes of at least 850 g, relative to 1 g of polymer.3. The polymer according to claim 1, wherein said polymer has aretention for a 0.9% aqueous NaCl-solution of at least 33 g/g polymerand a swelling pressure after 20 minutes of at least 900 g, relative to1 g of polymer.
 4. The polymer according to claim 1, wherein saidpolymer has a swelling pressure after 20 minutes of at least 800 g and amaximum pressure decrease of 20% after 2 hours, relative to the valueafter 20 minutes.
 5. The polymer according to claim 1, wherein saidpolymer has a maximum extractable portion of 3.5%-wt after 16 hours. 6.The polymer according to claim 1, wherein said water-soluble polymer isat least one polymer selected from the group consisting of a syntheticpolymer or copolymer, a natural polymer, and a derivative of a naturalpolymer.
 7. A process for producing the polymer of claim 1, comprisingmixing said components a)-d) to form an aqueous solution which has amaximum monomer content of 30% by weight, relative to the totalsolution; neutralizing said unsaturated, polymerizable, acidgroup-containing monomer a) with at least one base to the extent of5.0-30 mol-% prior to polymerization; initiating the polymerization ofsaid monomer (a) and monomer b) under radical conditions at atemperature ranging from 5-30° C. and continuing the polymerizationadiabatically; coarsely grinding and comminuting the resulting polymergel: neutralizing said polymer gel with at least one base, saidneutralized polymer gel having a content of polymerized, neutralized,acid group-containing monomer units of at least 50 mol-%; drying saidneutralized polymer gel to a water content of 5-20%-wt.; and reactingsaid dried, neutralized polymer gel at a temperature in the range of140-240° C. with at least one bi- or multifunctional crosslinkerreactive with acid groups.
 8. The process according to claim 7 whereinmonomer a) is neutralized to the extent of 10-20 mol-%, the radicalpolymerization is effected at temperatures of 8-15° C., and theneutralized polymer gel is dried to a water content of ≦10%-wt.
 9. Theprocess according to claim 7 wherein olefinically unsaturated,polymerizable mono- and/or dicarboxylic acids are used as unsaturated,polymerizable, acid group-containing monomer a).
 10. The processaccording to claim 9 wherein acrylic acid is used as said unsaturated,polymerizable, acid group-containing monomer a).
 11. The processaccording to claim 7 wherein in addition to the acid group-containingmonomer a) a water-soluble, unsaturated, polymerizable monomer b) isused.
 12. The process according to claim 11 wherein acrylamide,methacrylamide, N-alkylated (meth)acrylamides, N-methylol(meth)acrylamide, N-vinyl amides, N-vinyl formamide, N-vinyl acetamide,and N-vinyl-N-methylacetamide, N-vinyl-N-methylformamide,vinylpyrrolidone, hydroxyalkyl (meth)acrylates, hydroxyethyl acrylateand (meth)acrylic acid esters of polyethylene glycol monoallyl ether,and allyl ethers of polyethylene glycols, are used as the water-soluble,unsaturated, polymerizable monomer b).
 13. The process according toclaim 7 wherein a water-insoluble or sparingly water-soluble monomer b)is used.
 14. The process according to claim 13 wherein acrylic acid andmethacrylic acid esters, ethyl acrylate and methyl acrylate, vinylacetate, and styrene are used as water-insoluble or sparinglywater-soluble monomers b), in an amount of up to 10%-wt., relative tothe amount of all monomers.
 15. The process according to claim 7 whereinthe comminution is carried out by using an extruder having a terminalbreaker plate, said terminal breaker plate openings having a diameter inthe range of 2-20 mm.
 16. The process according to claim 15 wherein theopenings of the breaker plate have a diameter of 5-15 mm.
 17. A methodof absorbing moisture, comprising contacting an article comprising thepolymer of claim 1 with moisture.
 18. The method of claim 17, whereinsaid article is a sanitary article.
 19. The method of claim 17, whereinsaid article is a diaper.
 20. The method of claim 17, wherein saidarticle is a sanitary napkin.
 21. The method of claim 17, wherein saidarticle is an incontinence article.
 22. The method of claim 17, whereinsaid article is a current-conducting and light-transmitting cable. 23.The method of claim 17, wherein said article is a soil conditioner. 24.The method of claim 17, wherein said article is an artificial soil forplant cultivation.
 25. The method of claim 17, wherein said article is apackaging material.
 26. The method of claim 17, wherein said article isa depot material for the controlled release of active substances. 27.The method of claim 19, wherein said diaper optionally contains fluff,and the weight percent of said polymer of claim 1 is 15-100% by weightrelative to the combined weight of said polymer and fluff.
 28. Themethod of claim 19, wherein said diaper optionally contains fluff, andthe weight percent of said polymer of claim 1 is 30-70% by weightrelative to the combined weight of said polymer and fluff.